Application: Measuring the wall thickness of hollow metal turbine blades.

Background: Many turbine blades used in aircraft engines and other high-performance systems are hollow to permit the circulation of coolant within the blade. Core shifts during casting, faulty machining, or normal surface wear during operation can cause blade wall thickness to fall below acceptable limits. Mechanically measuring wall thickness is usually not possible without destroying the blade. However, with proper transducers and instrumentation this measurement can often be done ultrasonically.

Equipment: Depending on the geometry of the blade and the wall thickness, there are three types of ultrasonic gaging systems that are recommended:
Model 38DL PLUS or 35DL (with waveform display option) thickness gage with immersion or delay line transducers. The gage waveform display assists in monitoring waveforms and aids in transducer alignment.
Model 5072PR pulser-receivers with immersion or delay line transducers, reading pulse transit time from an oscilloscope or a PC-based system.

Procedure: The exact measurement system that is recommended for a given turbine blade application is defined on the basis of customer requirements and the acoustic properties of the blade. The following are basic considerations:

Transducer Type: Both delay line and immersion transducers are used in turbine blade applications. Delay line transducers are usually easy to position on the surface of a blade, while immersion transducers usually require scope monitoring of the waveform to verify alignment. However, the curvature of turbine blades can make it impossible to properly couple delay line transducers into the concave side. The 0.125 in. (3 mm) delay line transducers (M203 and M208) generally couple onto concave radii down to about 8 in. (200 mm). With contouring of the delay line it is possible, in some cases, to couple onto sharper radii, but in general sharply curved surfaces, particularly the leading edge of blades, are better measured with immersion transducers. The 20MHz V316-B in a B-120 bubbler provides a convenient handheld assembly for immersion transducer measurement of blades. In some cases, it is also possible to use our V260-SM and V260-RM Sonopen focused delay line transducers to couple onto concave surfaces that cannot be measured with conventional delay lines.

Measurement Mode: Using delay line and immersion transducers, thickness measurements may be made in either Mode 2 (interface to first back echo) or Mode 3 (echo to echo following interface). Mode 3 provides better thin material resolution than Mode 2, but it is possible only if the points to be measured on the turbine blade produce multiple backwall echoes. If there is only a single useable backwall echo (due to curvature or attenuation) measurement must be done in Mode 2. The Model 38DL PLUS and 35DL gages can operate in either Mode 2 or Mode 3. Establish the optimum setup for a given turbine blade application using the aid of reference standards representing the range of thickness and geometries to be measured.

Thickness Range : In typical metal blades the minimum resolvable thickness for a 20MHz delay line or immersion transducer is approximately 0.006 in. (0.15 mm) in Mode 3, and 0.020 in. (0.5 mm) in Mode 2. If thinner measurements are required, higher frequencies can be used with pulser-receivers or special instruments. Most turbine blade measurements are done at 10MHz or 20MHz.

Dead Spots: Hollow turbine blades sometimes contain assorted structures inside the blade to direct the coolant flow or add strength to the blade. Generally, it is not be possible to obtain a backwall echo from points where these vanes or ribs are located because the structure disrupts the smooth inside surface necessary for a good reflection. In cases where these structures are closely spaced, a focused immersion transducer with a small spot size produces better backwall echoes than a delay line transducer. There are also some cases where sharply tapering blade thickness can create a situation where inside walls and outside walls are significantly non-parallel, which can cause echo distortion and potential measurement errors.
In all cases, determine the combination of transducer and instrument on the basis of tests on actual product samples. The wide variation in turbine blade geometries makes sample evaluation important.

Figures 1 - 3 show typical waveforms associated with turbine blade measurement. The gage display screen instantly provides the live ultrasonic waveform with gates and thickness readings. This is ideal for difficult applications or when setup parameters need closer examination.

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Figure 1: Mode 3 Measurement of a Turbine Blade

Figure 1 shows a Mode 3 measurement of a 0.041 in. (1.05 mm) thick concave turbine blade trailing edge using an M208 transducer. The curvature is of sufficiently large radius at the measurement point that the M208 couples well into the material, and the clean multiple echoes permit a Mode 3 measurement.

Figure 2: Mode 2 Measurement of a Turbine Blade

Figure 2 shows a Mode 2 measurement of a 0.070 in. (1.8 mm) thick convex turbine blade wall, also using an M208 transducer. Here the presence of internal ribs damps the echoes somewhat, and in the absence of clean multiple echoes a Mode 2 measurement is recommended.

Figure 3 : Immersion Technique to Measure Curved Convex Blade Section

Figure 3 shows an immersion technique using a V316-BB 20MHz immersion transducer with a B-120 handheld bubbler to check a thicker (0.051 in./1.3 mm) sharply curved (1 in./25 mm radius) convex blade section, where the sharp convex curvature made coupling a delay line transducer difficult. In this type of measurement, especially on concave surfaces, it is essential for the operator to monitor waveforms to insure optimum transducer alignment.

Note: In addition to the standard line of immersion and delay line transducers, we offer three special low-profile 20MHz delay line transducers for measurement of turbine blade thickness in multi-blade assemblies where the space between blades is limited and access is difficult. The M2054 is a 20MHz delay line transducer just 0.27 in./6.75 mm thick on a 3 in./75 mm handle. The M2055 is similar with a transducer/delay line assembly that is 0.40 in./10 mm thick. The V2034 has a 0.40 in./10 mm head on a 6 in./300 mm angled handle. Outline drawings of these transducers are available on request.